Forged aluminum alloy and manufacturing method for the same

ABSTRACT

To provide a forged aluminum alloy excellent in creep characteristics and a manufacturing method for the forged aluminum alloy. 
     A forged aluminum alloy contains Si: 0.10-0.25 mass %, Fe: 0.9-1.3 mass %, Cu: 1.9-2.7 mass %, Mg: 1.3-1.8 mass %, Zn: 0.10 mass % or less, Ni: 0.9-1.2 mass %, and Ti: 0.01-0.1 mass %, with the balance being Al and inevitable impurities, in which the total content of Fe and Ni is 2.2 mass % or less, the total content of Mn, Cr, and Zr is 0.20 mass % or less, the average equivalent circle diameter of an intermetallic compound is 4.5 μm or less, and variation of distance between the intermetallic compounds in the ST direction is 2.3 or less.

TECHNICAL FIELD

The present invention relates to a forged aluminum alloy and a manufacturing method for the forged aluminum alloy used for manufacturing a forged product made of an aluminum alloy excellent in the heat resistance property.

BACKGROUND ART

Traditionally, various proposals have been made for a forged aluminum alloy. For example, in JP-A No. 2017-214655 (Patent Literature 1), there is proposed an aluminum alloy containing Si: 0.1-2.0 wt %, Fe: 1.0-2.0 wt %, Cu: 2.0-6.0 wt %, Mg: 1.0-3.0 wt %, Ni: 3.0 wt % or less, and Ti: 0.01-0.2 wt %, with Si+Fe+Mg≤3.1 wt %, the balance being Al and inevitable impurities, and having electric conductivity of 25.0% IACS or more and 40.0% IACS or less.

SUMMARY OF THE INVENTION

In general, a majority of the rotational components and the direct acting components of an engine, a compressor, a turbo-charger impellor, and the like are used continuously at high temperature. Therefore, with respect to a forged aluminum alloy that is a raw material of these components, the creep characteristics are required particularly. The aluminum alloy proposed in Patent Literature 1 is classified into an AA2618 aluminum alloy excellent in the heat resistance property. It is expressed that the aluminum alloy proposed in Patent Literature 1 has the tensile strength at 250° C. higher than that of JIS 4032 alloy. However, with respect to the aluminum alloy proposed in Patent Literature 1, there was a room for further improvement in enhancing the creep characteristics.

The object of the present invention is to provide a forged aluminum alloy excellent in creep characteristics and a manufacturing method for the forged aluminum alloy.

As a result of sincere research and development to provide excellent creep characteristics for a forged aluminum alloy formed of an AA2618 aluminum alloy, the present inventor found out that the problem described above could be solved by controlling the total content of Fe and Ni and the total content of Mn, Cr, and Zr and controlling the size of the intermetallic compound and uniformity of dispersion of the intermetallic compound, and completed the present invention.

Specifically, the forged aluminum alloy related to the present invention is a forged aluminum alloy configured to contain Si: 0.10-0.25 mass %, Fe: 0.9-1.3 mass %, Cu: 1.9-2.7 mass %, Mg: 1.3-1.8 mass %, Zn: 0.10 mass % or less, Ni: 0.9-1.2 mass %, and Ti: 0.01-0.1 mass %, with the balance being Al and inevitable impurities, in which the total content of Fe and Ni is 2.2 mass % or less, the total content of Mn, Cr, and Zr is 0.20 mass % or less, the average equivalent circle diameter of an intermetallic compound is 4.5 μm or less, and variation of distance between the intermetallic compounds in the ST direction is 2.3 or less.

Also, the manufacturing method of a forged aluminum alloy related to the present invention is a manufacturing method of the forged aluminum alloy having the alloy composition described above, and is configured to include the steps of cooling molten metal having the alloy composition with a cooling rate of 1.2° C./sec or more to cast an ingot, and manufacturing a forged aluminum alloy by subjecting the ingot to homogenization treatment, hot forging, solution treatment, quenching, and temper aging treatment.

Advantageous Effects of Invention

The forged aluminum alloy related to the present invention is excellent in creep characteristics.

Also, the manufacturing method for a forged aluminum alloy related to the present invention can manufacture a forged aluminum alloy excellent in creep characteristics.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic drawing explaining how the temperature of the molten metal is measured in the case of the semi-continuous casting method (DC casting method (Direct Chill casting process)).

FIG. 2 is a schematic cross-sectional view explaining how to obtain a linear approximate expression of the mold temperature and the cooling rate of the molten metal in the mold casting method.

FIG. 3 is a graph illustrating an example of the mold temperature, the cooling rate of the molten metal, and the linear approximate expression. In the drawing, the horizontal axis expresses the mold temperature (° C.), and the vertical axis expresses the cooling rate of the molten metal (° C./sec).

FIG. 4 is an explanatory drawing explaining a mold, an ingot taken out from the mold, and a portion for manufacturing a sample for a forging test from the ingot.

FIG. 5 is a plan view of a forged material manufactured by hot forging. The hatching portion in the drawing illustrates the cut-out position of a test specimen used for the tensile test and the creep test.

FIG. 6 is a side view of a forged material manufactured by hot forging. The hatching portion in the drawing illustrates the cut-out position of a test specimen used for the tensile test and the creep test.

FIG. 7 is a graph illustrating the mold temperature, the cooling rate of the molten metal, and the linear approximate expression. In the drawing, the horizontal axis expresses the mold temperature (° C.), and the vertical axis expresses the cooling rate of the molten metal (° C./sec).

FIG. 8 is a graph illustrating a relation between the total content (mass %) of Mn, Cr, and Zr and the total content (mass %) of Fe and Ni with respect to an example and a comparative example. In the drawing, the example is plotted by “♦”, and the comparative example is plotted by “□”.

FIG. 9 is a graph illustrating a relation between the average equivalent circle diameter of an intermetallic compound (μm) and variation of the distance between intermetallic compounds in the ST direction with respect to the example and the comparative example. In the drawing, the example is plotted by “♦”, and the comparative example is plotted by “□”.

FIG. 10 is a graph illustrating a relation between dispersion of the distance between intermetallic compounds in the ST direction and the rupture time (hr) of the creep test with respect to the example and the comparative example. In the drawing, the example is plotted by “♦”, and the comparative example is plotted by “□”.

FIG. 11 is a graph illustrating a relation between the average equivalent circle diameter (μm) of an intermetallic compound and the rupture time (hr) of the creep test with respect to the example and the comparative example. In the drawing, the example is plotted by “♦”, and the comparative example is plotted by “□”.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be hereinafter explained referring to the drawings from time to time. Also, since the drawings referred to in the explanation below schematically illustrates an embodiment of the present invention, there are cases that the scale, distance, positional relationship, and the like of respective members are exaggerated or that illustration of a part of the member is omitted. “-” described in the present description is used in the meaning of having the numerical figures described before and after thereof as the lower limit value and the upper limit value. An upper limit value or a lower limit value described stepwise in the present description may be replaced by another upper limit value or lower limit value described stepwise, and may be replaced by numerical figures shown in an example.

[Forged Aluminum Alloy]

First, an embodiment of a forged aluminum alloy will be explained.

The forged aluminum alloy related to the present embodiment is provided with a prescribed alloy composition as an aluminum alloy, and is used for manufacturing a forged product (an aluminum alloy component). The forged aluminum alloy is provided with such alloy composition and structural features as hereinafter described.

Specifically, the forged aluminum alloy contains Si: 0.10-0.25 mass %, Fe: 0.9-1.3 mass %, Cu: 1.9-2.7 mass %, Mg: 1.3-1.8 mass %, Zn: 0.10 mass % or less, Ni: 0.9-1.2 mass %, and Ti: 0.01-0.1 mass %, with the balance being Al and inevitable impurities, in which the total content of Fe and Ni is 2.2 mass % or less, the total content of Mn, Cr, and Zr is 0.20 mass % or less. Also, the forged aluminum alloy has such a configuration that the average equivalent circle diameter of an intermetallic compound is 4.5 μm or less, and variation of the distance between the intermetallic compounds in the ST direction is 2.3 or less. Further, the variation of the distance between the intermetallic compounds in the ST direction is applicable to all intermetallic compounds that can be observed.

(Alloy Composition)

The aluminum alloy formed of the alloy composition described above belongs to an AA2618 aluminum alloy. The AA2618 aluminum alloy is used for a structural member where the high-temperature properties are required. The alloy composition described above will be hereinafter explained. Also, these compositions are known generally as described in the Japanese Patent No. 6348466 and the Japanese Patent No. 5879181 for example, and have such actions as described below.

(Si: 0.10-0.25 Mass %)

Along with Mn, Si allows fine dispersal phases of an Al—Mn—Si system compound and the like to precipitate, improves the pinning effect of the dislocation, suppresses coarsening of the recrystallized grain during the solution treatment, and thereby improves the strength of the forged aluminum alloy. The Si content is made to be within the range of 0.10-0.25 mass %. When the Si content is less than 0.10 mass %, the effect of improving the strength cannot be secured sufficiently. When the Si content exceeds 0.25 mass %, a compound of Mg and Si and the like is formed, and the heat resistance property deteriorates. Also, the lower limit value of the Si content is preferably 0.13 mass %, and is more preferably 0.15 mass %. Further, the upper limit value of the Si content is preferably 0.23 mass %, and is more preferably 0.21 mass %.

(Fe: 0.9-1.3 Mass %)

Along with Ni, Fe forms an Fe—Ni system compound and the like, and improves the heat resistance property of the forged aluminum alloy. The Fe content is made to be within the range of 0.9-1.3 mass %. When the Fe content is less than 0.9 mass %, the effect of improving the heat resistance property cannot be secured sufficiently. When the Fe content exceeds 1.3 mass %, since Fe-system compounds of an Al—Fe system, an Al—Fe—Cu system, and the like dispersing within the matrix are formed extremely, the effect of improving the heat resistance property is deteriorated. Also, the lower limit value of the Fe content is preferably 1.0 mass %. Further, the upper limit value of the Fe content is preferably 1.2 mass %.

(Cu: 1.9-2.7 Mass %)

Cu contributes to improvement of the strength of the forged aluminum alloy at ordinary temperature and high temperature. Because of both actions of solid solution strengthening and precipitation strengthening, Cu is an element indispensable for securing the ordinary temperature proof stress and the heat resistance property required for an Al alloy mainly in the usage of the present invention. To be more specific, Cu chemically combines with Al and Mg at the time of the artificial temper aging treatment at high temperature, allows the G.P.B. zone, the S′ phase, and the like to precipitate finely and with high density, and improves the strength of an Al alloy after the artificial temper aging treatment. The Cu content is made to be within the range of 1.9-2.7 mass %. When the Cu content is less than 1.9 mass %, the effect of improving the strength cannot be secured sufficiently. When the Cu content exceeds 2.7 mass %, since the eutectic melting starting temperature lowers and the solution treatment temperature should be lowered, the solid solution amount into the matrix reduces, and the effect of improving the strength cannot be expected. The lower limit value of the Cu content is preferably 2.0 mass %, and is more preferably 2.1 mass %. Further, the upper limit value of the Cu content is preferably 2.6 mass %, and is more preferably 2.5 mass %.

(Mg: 1.3-1.8 Mass %)

Coexisting with Cu, Mg contributes to improvement of the strength of the forged aluminum alloy at ordinary temperature and high temperature. Because of both actions of solid solution strengthening and precipitation strengthening, Mg is an element indispensable for securing the ordinary temperature proof stress and the heat resistance property required for an Al alloy mainly in the usage of the present invention. To be more specific, Mg chemically combines with Al and Cu at the time of the artificial temper aging treatment at high temperature, allows the G.P.B. zone, the S′ phase, and the like to precipitate finely and with high density, and improves the strength of an Al alloy after the artificial temper aging treatment. The Mg content is made to be within the range of 1.3-1.8 mass %. When the Mg content is less than 1.3 mass %, the effect of improving the strength is small. When the Mg content exceeds 1.8 mass %, deformation resistance of the material increases in hot working such as forging, and the productivity drops. The lower limit value of the Mg content is preferably 1.4 mass %, and is more preferably 1.5 mass %. Further, the upper limit value of the Mg content is preferably 1.7 mass %, and is more preferably 1.6 mass %.

(Zn: 0.10 Mass % or Less)

Zn is an element contained often as the inevitable impurities. Further, Zn is also one of the additive elements that can improve the ordinary temperature strength and the high temperature strength of the forged aluminum alloy by the solid solution strengthening and the precipitation strengthening. However, Zn is particularly harmful with respect to the corrosion resistance property. Therefore, the Zn content is made to be 0.10 mass % or less. Since the effect of improving the ordinary temperature strength and the high temperature strength of the forged aluminum alloy can be secured sufficiently by adding Cu and Mg, it is allowable not to contain Zn (the Zn content may be 0 mass %). The upper limit value of the Zn content is preferably 0.09 mass %, is more preferably 0.08 mass %, and is furthermore preferably 0.05 mass %.

(Ni: 0.9-1.2 Mass %)

Along with Fe, Ni forms an Fe—Ni system compound and the like, and improves the heat resistance property of the forged aluminum alloy. The Ni content is made to be within the range of 0.9-1.2 mass %. When the Ni content is less than 0.9 mass %, the effect of improving the heat resistance property cannot be secured sufficiently. When the Ni content exceeds 1.2 mass %, since Ni-system compounds of an Al—Ni system, an Al—Ni—Cu system, and the like dispersing within the matrix are formed, the effect of improving the heat resistance property is deteriorated. Also, when the Ni content exceeds 1.2 mass %, the intermetallic compounds of a coarse Fe—Ni system and the like are formed. Therefore, in this case, a crack is liable to occur in hot working such as forging, and the productivity drops. The lower limit value of the Ni content is preferably 0.95 mass %, and is more preferably 1.0 mass %. Also, the upper limit value of the Ni content is preferably 1.18 mass %, and is more preferably 1.1 mass %.

(Ti: 0.01-0.1 Mass %)

Similarly to Zr, Ti is added in order to stably secure the fine grain structure. The Ti content is made to be within the range of 0.01-0.1 mass %. When the Ti content is less than 0.01 mass %, the effect of stabilizing the fine grain structure cannot be secured sufficiently. When the Ti content exceeds 0.1 mass %, gigantic Zr—Ti system compounds and the like are formed at the time of casting, and the strength drops. The lower limit value of the Ti content is preferably 0.04 mass %. The upper limit value of the Ti content is preferably 0.09 mass %.

(The Balance)

The balance is Al and inevitable impurities. The inevitable impurities are possibly contained inevitably deriving from raw material used and the like in the actual operation. As the inevitable impurities, Mn, Cr, Zr, V, and the like for example can be cited other than Zn described above. V does not cause any problem particularly as far as it is 0.05 mass % or less. Mn, Cr, and Zr will be described below. Also, Mn, Cr, Zr, and V do not inhibit the effect of the present invention not only in being contained as the inevitable impurities but also in being added positively as far as they do not exceed prescribed content and total content explained in the present description.

(Total Content of Fe and Ni: 2.2 Mass % or Less)

Although Fe and Ni improve the heat resistance property of the forged aluminum alloy, when the total content thereof exceeds 2.2 mass %, a great number of intermetallic compounds of the Fe—Ni system intermetallic compounds and the like come to be formed. Therefore, the area ratio and the number density of the intermetallic compound in a prescribed plane (cross section) become high. As the intermetallic compound appearing when the total content of Fe and Ni exceeds 2.2 mass %, the Al—Cr—Cu—Fe—Mn—Si system, Fe—Ni system, Mg—Si system, and Al—Cu—Fe system intermetallic compound, and the like can be cited. Also, when the total content of Fe and Ni exceeds 2.2 mass %, bias comes to occur in dispersion of the intermetallic compounds due to the segregation. Also, by being thereby affected, the variation in the distance between the intermetallic compounds in the ST direction becomes large. Concrete range of the numerical value of the variation of the distance between the intermetallic compounds in the ST direction will be described below. The fact that the variation of the distance between the intermetallic compounds in the ST direction is large means that the dispersion of the intermetallic compounds is not uniform, and the creep characteristics thereby deteriorate. Therefore, the total content of Fe and Ni is made to be 2.2 mass % or less. The upper limit value of the total content of Fe and Ni is preferably 2.1 mass %, and is more preferably 2.0 mass %. Also, as the prescribed plane (cross section), the cross section perpendicular to the metal flow (L direction) of the forged aluminum alloy or the test specimen can be cited for example. Further, the ST direction means the thickness direction along which the aluminum alloy is deformed most by the forging work.

(Total Content of Mn, Cr, and Zr: 0.20 Mass % or Less)

As described above, Mn, Cr, and Zr are contained as the inevitable impurities. Mn, Cr, and Zr miniaturize the metal structure (microstructure) of the forged aluminum alloy, and suppress recrystallization. However, when the total content of Mn, Cr, and Zr exceeds 0.20 mass %, Mn, Cr, and Zr segregate at the time of casting or assist formation and growth of nuclei of the crystallized grain, and therefore bias comes to occur in dispersion of the intermetallic compounds. Thus, for example, the variation of the distance between the intermetallic compounds in the ST direction becomes large. Also, since the dispersion of the intermetallic compounds is not uniform, the creep characteristics deteriorate. Therefore, the total content of Mn, Cr, and Zr is made to be 0.20 mass % or less. The upper limit value of the total content of Mn, Cr, and Zr is preferably 0.15 mass %.

(Microstructure) (Average Equivalent Circle Diameter of Intermetallic Compound: 4.5 μm or Less)

When the average equivalent circle diameter exceeds 4.5 μm, the creep characteristics of the intermetallic compound deteriorate. Therefore, the average equivalent circle diameter of the intermetallic compound is made to be 4.5 μm or less. The average equivalent circle diameter of the intermetallic compound is preferably 4.2 μm or less, and is more preferably 4.0 μm or less. The average equivalent circle diameter of the intermetallic compound is largely affected by the cooling rate of the molten metal in casting the ingot. The cooling rate of the molten metal in casting the ingot will be explained in the item of the manufacturing method.

(Variation of Distance Between Intermetallic Compounds in ST Direction: 2.3 or Less)

The variation (standard deviation) of the distance between the intermetallic compounds in the ST direction is obtained by calculating and evaluating the variation of the shortest distance between the intermetallic compounds adjacent to each other in the ST direction in the observed cross section. Specifically, this variation expresses the uniformity of the dispersion of the intermetallic compounds in the ST direction. As this variation is smaller, it means that the intermetallic compounds are dispersed more uniformly, and the forged aluminum alloy can be said to have an excellent microstructure. The variation of the distance between the intermetallic compounds in the ST direction can be obtained by the expression (1).

$\begin{matrix} \left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack & \; \\ {{S = \sqrt{\frac{1}{n}{\sum_{i = 1}^{n}\left( {x_{i} - \overset{¯}{x}} \right)^{2}}}}{\text{S:}\mspace{14mu}{Standard}\mspace{14mu}{deviation}}{\text{n:}\mspace{14mu}{Total}\mspace{14mu}{number}\mspace{14mu}{of}\mspace{14mu}{data}}{{x_{i}:}\mspace{14mu}{Value}\mspace{14mu}{of}\mspace{14mu}{each}\mspace{14mu}{data}}{{\overset{\_}{x}\text{:}}\mspace{14mu}{Average}\mspace{14mu}{of}\mspace{14mu}{data}}} & (1) \end{matrix}$

When the variation of the distance between the intermetallic compounds in the ST direction exceeds 2.3, since the uniformity of the dispersion of the intermetallic compounds is low (namely the intermetallic compounds are unevenly distributed), the creep characteristics deteriorate. Therefore, the variation of the distance between the intermetallic compounds in the ST direction is made to be 2.3 or less. This variation is preferably 2.13 or less, and is more preferably 2.02 or less.

(Observation of Microstructure (Average Equivalent Circle Diameter, and the Like))

The average equivalent circle diameter of the intermetallic compounds, the distance between the intermetallic compounds in the ST direction, and so on can be grasped by observing the microstructure in a manner described below.

For example, in observing the microstructure, a test specimen of approximately 10 mm square is cut out from a forged material having been subjected to T61 temper treatment, mechanical polishing and buffing are performed, and two to three visual fields of the metal structure of the cross section perpendicular to the metal flow (L direction) are thereafter observed with the magnification of 100 times using an optical microscope (ECLIPSE MA200 made by Nikon Corporation). From the photos of the observed structure, the grain diameter in the ST direction is measured using the section method, and the average equivalent circle diameter, the ratio of the area occupying the visual field, the number density, and so on are calculated with respect to the intermetallic compound by an image analysis using analysis software.

Also, all distances between the intermetallic compounds that can be observed are observed and measured with respect to the ST direction, and the variation of the distance is evaluated by the standard deviation. To the intermetallic compounds that can be observed, for example, those having the grain diameter of 0.4 μm or more correspond.

Although WinROOF2018 made by Mitani Corporation could be used for the analysis software in the example, the analysis software is not limited to it.

As described above, the forged aluminum alloy can secure excellent creep characteristics by employing the prescribed alloy composition, making the total content of Fe and Ni and the total content of Mn, Cr, and Zr to be a prescribed value or less respectively, and making the average equivalent circle diameter of the intermetallic compounds and the variation of the distance between the intermetallic compounds in the ST direction to be a prescribed value or less respectively.

[Manufacturing Method]

Next, an embodiment of the manufacturing method for a forged aluminum alloy will be explained. Also, in the explanation below on the manufacturing method, detailed explanation will be omitted with respect to the items already explained.

The manufacturing method for a forged aluminum alloy related to the present embodiment is a manufacturing method for manufacturing a forged aluminum alloy having the alloy composition described above.

The present manufacturing method manufactures a forged aluminum alloy by cooling molten metal having the alloy composition described above with the cooling rate of 1.2° C./sec or more to cast an ingot and subjecting the ingot to homogenization treatment, hot forging, solution treatment, quenching, and temper aging treatment within a temperature range having been set beforehand.

(Cooling Rate of Molten Metal: 1.2° C./Sec or More)

The cooling rate of the molten metal in casting an ingot largely affects the size of the intermetallic compound (Fe—Ni system intermetallic compound, and the like for example). The cooling rate of the molten metal is made to be 1.2° C./sec or more. When the cooling rate of the molten metal is less than 1.2° C./sec, since it takes too much time for solidifying the molten metal, Fe, Ni, and the like having been solid-dissolved precipitate to the grain boundary, and allow the intermetallic compound to grow largely. Therefore, the size of the intermetallic compound becomes large. The cooling rate of the molten metal is preferably 1.8° C./sec or more, is more preferably 3° C./sec or more, and is furthermore preferably 10° C./sec or more.

(Measurement and Calculation of Cooling Rate of Molten Metal)

Measurement and calculation of the cooling rate of the molten metal can be executed by methods cited in <1> to <3> below for example in the case of the mold casting method and the case of the DC casting method. As the condition of the cooling rate of the molten metal in casting an ingot, the present manufacturing method can use the cooling rate of the molten metal calculated beforehand by the method cited in <1> to <3>.

<1> Measuring/Calculating Method of Cooling Rate of Molten Metal in the Case of Mold Casting Method

As the measuring/calculating method of the cooling rate of the molten metal in the case of the mold casting method, it can be cited to directly measure the temperature of the molten metal having been casted by a digital recorder connected to a thermocouple, and to calculate the cooling rate of the molten metal from the result.

<2> Measuring/Calculating Method of Cooling Rate of Molten Metal in the Case of DC Casting Method

As the measuring/calculating method of the cooling rate of the molten metal in the case of DC casting method, following manner can be cited. Here, FIG. 1 is a schematic drawing explaining how the temperature of the molten metal is measured in the case of the DC casting method.

As illustrated in FIG. 1, a thermocouple 12 is installed in a DC casting apparatus 11. The thermocouple 12 is apart by approximately 10 mm from a mold 13 of the DC casting apparatus 11, and is installed at a position 16 where a molten metal 14 is molten aluminum before being solidified to become an ingot 15 (a position slightly lower than the surface of the molten metal 14 inside the mold 13). Also, the mold 13 is a water-cooled type mold, and is configured to eject the water W toward the molten aluminum from the inside of the mold 13. A wire 18 is fixed to the bottom part of a casting table 17. The wire 18 is fixed so that the longitudinal direction becomes the direction parallel to the drawing down direction of the casting table 17. Also, by fixing the thermocouple 12 to this wire 18, the thermocouple 12 is configured not to shift in the horizontal direction. The thermocouple 12 is connected to a digital recorder 19, and directly measures the temperature of the molten metal 14. Specifically, the thermocouple 12 is drawn down by the casting table 17 along with the wire 18 that is fixed to the bottom part of the casting table 17, and directly and continuously measures the temperature until the molten metal 14 is solidified to become the ingot 15. Also, the cooling rate (° C./sec) is calculated from the change of the temperature with respect to a unit time when the temperature of the molten metal lowers to the solid phase line (550° C.).

<3> Another Embodiment of Measuring/Calculating Method of Cooling Rate of Molten Metal in the Case of Mold Casting Method

Since the methods cited in <1> and <2> described above directly measure the temperature of the molten metal using the thermocouple, precise cooling rate of the molten metal can be measured and calculated. Here, in the case of the mold casting method, other than the method cited in <1>, it is also possible to calculate (estimate) the cooling rate of the molten metal from the mold temperature and a correlation expression (linear approximate expression) having been set beforehand as explained below.

The linear approximate expression is obtained as follows for example. Here, FIG. 2 is a schematic cross-sectional view explaining how to obtain the linear approximate expression of the mold temperature and the cooling rate of the molten metal in the mold casting method.

As shown in FIG. 2, a thermocouple 24 is disposed so as to be capable of measuring the temperature of the center position 23 of a space 22 to which the molten metal within a mold 21 is made to flow in. Next, the mold 21 is heated to a prescribed temperature by a gas burner, and the mold temperature around an opening section 25 of the mold 21 is measured by a contact type thermometer. Thereafter, the molten metal (720° C. for example) meltingly regulated to have the alloy composition described above is made to flow into the mold 21 to manufacture an ingot. The temperature of the molten metal is recorded continuously by a digital recorder 26 to which the thermocouple 24 is connected. Also, the cooling rate (° C./sec) is calculated by the change of the temperature per the unit time of the time when the temperature of the molten metal lowers to the solid phase line (550° C.). This process is executed for three points of 200° C., 246° C., and 296° C. of the mold temperature for example, and the cooling rate of the molten metal at each mold temperature is calculated.

In the case of 200° C. of the mold temperature, the cooling rate when the molten metal lowers to the solid phase line is 3.25° C./sec for example.

In the case of 246° C. of the mold temperature, the cooling rate when the molten metal lowers to the solid phase line is 2.7° C./sec for example.

In the case of 296° C. of the mold temperature, the cooling rate when the molten metal lowers to the solid phase line is 2.2° C./sec for example.

Also, FIG. 3 is a graph illustrating an example of the mold temperature, the cooling rate of the molten metal, and the linear approximate expression. As shown in FIG. 3, the linear approximate expression is obtained from three points shown by the × mark (three cooling rates of the molten metal exemplified above). In the example shown in FIG. 3, the linear approximate expression becomes y=−0.0109x+5.4185. Also, in the expression, y is the cooling rate (° C./sec) of the molten metal, and x is the mold temperature (° C.). In estimating the cooling rate of the molten metal, the linear approximate expression thus obtained can be used. The estimated value of the cooling rate of the molten metal calculated by this linear approximate expression is generally the same as the cooling rate calculated from the temperature of the molten metal measured by the directly measuring method of <1> described above.

Although the measuring/calculating methods of the cooling rate of the molten metal were explained above, the measuring/calculating method of the cooling rate of the molten metal is not limited to them in the present embodiment, and may be one measured and calculated by a method other than those explained above.

(Other Treatments in the Present Manufacturing Method)

Other treatments in the present manufacturing method namely melting regulation of the alloy composition of the molten metal, homogenization treatment, hot forging, solution treatment, quenching, and temper aging treatment can be executed by a general condition executed for the forged aluminum alloy formed of the AA2618 aluminum alloy for example. For example, melting regulation of the alloy composition of the molten metal can be executed at 700 to 760° C. The homogenization treatment can be executed at 450 to 550° C. The hot forging can be executed at 300 to 355° C. The solution treatment can be executed at 500 to 550° C. Although the quenching can be executed using the water or oil with the temperature of 90 to 100° C., it is also possible to be executed using the water or oil of the ordinary temperature (approximately 25° C.). The temper aging treatment can be executed at 170 to 220° C. Namely, although the present manufacturing method can be executed by the T6 treatment or the T61 treatment, it is executed preferably by the T61 treatment. The present manufacturing method can manufacture the forged aluminum alloy related to the present embodiment by employing such manufacturing conditions. Also, the present manufacturing method is not limited to the general conditions exemplified here.

[Aluminum Alloy Component]

The forged aluminum alloy related to the present embodiment is forged (hot forging, cold forging) to obtain a near net shape and is thereafter subjected to machining, and thereby the rotational component and the direct moving component (aluminum alloy component) of an engine, a compressor, a turbocharger impellor, and the like can be manufactured.

EXAMPLE

Next, the forged aluminum alloy and the manufacturing method for the forged aluminum alloy related to the present invention will be explained concretely by examples. FIG. 4 is an explanatory drawing explaining a mold, an ingot taken out from the mold, and a portion for manufacturing a sample for a forging test from the ingot. FIG. 5 is a plan view of a forged material manufactured by hot forging. FIG. 6 is a side view of the forged material manufactured by hot forging. The hatching portions in FIG. 5 and FIG. 6 illustrate the cut-out position of a test specimen used respectively for the tensile test and the creep test.

With the molten metal (720° C.) of the AA2618 aluminum alloy regulated to have the alloy compositions shown in No. 1 to No. 9 of Table 1, ingots were manufactured by the mold casting method using a copper mold or the DC casting method. The mass of the ingot by the copper mold (described simply as “mold” in Table 1) thus manufactured was approximately 8.5 kg, and the mass of the ingot by the DC casting method (described simply as “DC” in Table 1) was approximately 17 kg and approximately 42 kg. Also, the present example is not limited to the general conditions exemplified here, and is also applicable similarly in an ingot with the mass of 2 t or more.

Also, as illustrated in FIG. 4, an ingot 42 taken out from a mold 41 was cut and machined, and a sample 43 with 100 mm of the diameter×120 mm of the height was manufactured from the generally center position of the ingot.

Next, the sample 43 was subjected to homogenizing heat treatment of 520° C.×20 hrs in an air furnace, and was thereafter cooled to the room temperature.

Thereafter, the sample 43 was heated to 300 to 340° C. in the air furnace, was thereafter taken out from the furnace, and was hot-forged to have the height of 30 mm (the diameter was approximately 200 mm) as shown in FIG. 5 and FIG. 6 by an oil-hydraulic forging press to manufacture a forged material 56.

Thereafter, the forged material 56 was subjected to the solution treatment of 530° C.×6 hrs in the air furnace, and was thereafter quenched in the boiling water.

Then, the forged material 56 was subjected to the artificial temper aging treatment of 197° C.×22 hrs in the air furnace, and was made to be a test material of the T61 treatment.

From the test material thus manufactured, a test specimen for microstructure observation described below, a test specimen for the tensile test, and a test specimen for the creep test were cut out respectively, and were used for observation and testing. Also, the test specimen for the tensile test and the test specimen for the creep test were cut out so as to become the hatching portion of the L direction illustrated in FIG. 5 and the hatching portion of the center part in the ST direction illustrated in FIG. 6. Here, the L direction means the metal flow direction of the test material. In the present example and the comparative example, the longitudinal direction (the radial direction) of the test material in a plan view corresponds to the L direction. Also, the ST direction means the thickness direction along which the aluminum alloy was subjected to deformation most by the forging work. In the present example and the comparative example, the thickness direction of the test material corresponds to the ST direction.

Here, in the case of the mold casting method, the mold temperature was measured by the contact type thermometer immediately before the molten metal was made to flow into the copper mold. The mold temperature is shown in Table 1. Also, with respect to the DC casting method, since the water-cooled mold was used, the mold temperature was not measured.

The cooling rate (° C./sec) was calculated from the change of the temperature per the unit time when the temperature of the molten metal lowers to the solid phase line (550° C.). Here, based on the content explained in <3> of the embodiment, the cooling rate of the molten metal in the mold casting method was calculated with respect to three points of 200° C., 246° C., and 296° C. of the mold temperature.

As a result, the cooling rate when the molten metal (720° C.) lowered to the solid phase line was calculated to be 3.25° C./sec in the case of the mold temperature of 200° C., 2.7° C./sec in the case of the mold temperature of 246° C., and 2.2° C./sec in the case of the mold temperature of 296° C.

FIG. 7 is a graph illustrating the mold temperature, the cooling rate of the molten metal, and the correlation expression (the linear approximate expression). Three cooling rates of the molten metal calculated are shown by the × marks in FIG. 7. Also, from these three cooling rates of the molten metal, the linear approximate expression was obtained. As shown in FIG. 7, the linear approximate expression was y=−0.0109x+5.4185. Also, y is the cooling rate (° C./sec) of the molten metal, and x is the mold temperature (° C.). The cooling rate of the molten metal in the mold casting method can be calculated (estimated) using the mold temperature and this linear approximate expression, and the cooling rate of the molten metal in the mold casting method (casting die) shown in Table 1 is the value calculated by this linear approximate expression. The estimate value of the cooling rate of the molten metal calculated by this linear approximate expression was generally the same as the cooling rate calculated from the temperature of the molten metal measured by the directly measuring method explained in <1> of the embodiment.

According to the graph and the linear approximate expression of FIG. 7, although the cooling rate of the molten metal was estimated to be 1.87° C./sec in the case of the mold temperature of 326° C., it was rounded to one decimal place and was expressed in Table 1 as 1.9° C./sec.

In a similar manner, although the cooling rate of the molten metal was estimated to be 1.80° C./sec in the case of the mold temperature of 332° C., it was rounded to one decimal place and was expressed in Table 1 as 1.8° C./sec.

In a similar manner, although the cooling rate of the molten metal was estimated to be 1.76° C./sec in the case of the mold temperature of 336° C., it was rounded to one decimal place and was expressed in Table 1 as 1.8° C./sec.

In a similar manner, although the cooling rate of the molten metal was estimated to be 1.13° C./sec in the case of the mold temperature of 393° C., it was rounded to one decimal place and was expressed in Table 1 as 1.1° C./sec.

When the mold temperature is 200° C., although the cooling rate of the molten metal was 3.25° C./sec according to the actually measured value described above and the graph and the linear approximate expression of FIG. 7, similarly to the above, it was rounded to one decimal place and was expressed in Table 1 as 3.3° C./sec.

On the other hand, the cooling rate of the molten metal in the DC casting method was measured and calculated based on the content explained in <2> of the embodiment referring to FIG. 1. The cooling rate of the molten metal in the case of the DC casting method is shown in Table 1.

(Observation of Microstructure)

Observation of the microstructure of the test material of the T61 treatment manufactured was executed as described below.

A test specimen of approximately 10 mm square was cut out from the test material, was subjected to mechanical polishing and buffing, and two to three visual fields of the metal structure of the cross section perpendicular to the metal flow (L direction) of the test material were thereafter observed and photographed with the magnification of 100 times using the optical microscope (ECLIPSE MA200 made by Nikon Corporation). The photos of the structure observed and photographed were analyzed using the analysis software (WinROOF2018 made by Mitani Corporation). The grain diameter (μm) was measured, and the average equivalent circle diameter (μm), the ratio (%) of the area occupying the visual field, and the number density (piece/mm²) were calculated with respect to the intermetallic compound.

Also, all distances between the intermetallic compounds whose average equivalent circle diameter was 0.4 μm or more and which could be observed were measured with respect to the ST direction, and the variation of the distance was evaluated by the standard deviation. The standard deviation was calculated by the expression (1) explained in the embodiment.

The result of them are shown in Table 2.

(Tensile Test, Creep Test)

A tensile test specimen (specimen TP) with approximately 85 mm of the total length, 25 mm of the length of the parallel section, and 6.35 mm of the diameter of the parallel section in accordance with ASTM E8 and ASTM B357 was manufactured from the position shown in the hatching portion in FIG. 5 and FIG. 6, and the tensile test was executed.

Also, a creep test specimen (specimen TP) with approximately 90 mm of the total length, 40 mm of the length of the parallel section, and 6 mm of the diameter of the parallel section was manufactured from the similar position, the creep test by JIS Z 2271: 2010 was executed until the test specimen was broken at 180° C. and the load stress of 220 MPa. As the result of the creep test, one with the rupture time of 220 hrs or more was made to have passed (∘), and one with the rupture time of less than 220 hrs was made not to have passed (x). Along with the temperature and the load stress of the creep test, the result of the creep test namely the creep characteristic (rupture time (hr)) is shown in Table 2.

(Larson-Miller Parameter (LMP))

When the creep characteristics are to be applied to the design criteria, the stress causing the creep rate of 0.01% per 1,000 hrs and the 100,000 hrs rupture pulsating stress are commonly used. However, it takes a long time for obtaining these values. Therefore, as a method for estimating the creep characteristics of long time from the test result of a short time, it is common to use LMP (Larson-Miller parameter) (“Aluminum zairyou-no kiso-no kougyou gijutu (Basic Industrial Technology of Aluminum Material)”, published by Japan Aluminum Association, P. 307). LMP is defined for example by LMP=T(20+Log(tr)) where T: temperature (absolute temperature K), tr: rupture time (hr). As this expression shows, the creep characteristics of a low temperature and long time can be estimated from the creep test result of a high temperature and a short time. LMP calculated from this expression is shown in Table 2 along with the result of the creep test.

TABLE 1 Cooling rate of Alloy composition (mass %) (balance: Al and inevitable impurities) Mold molten Mn + Cr + Casting temperature metal No. Si Fe Cu Mn Mg Cr Zn Ti Ni Zr Fe + Ni Zr method (° C.) (° C./sec) 1 0.22 1.0 2.6 0.033 1.5 0.034 0.05 0.070 1.1 0.002 2.10 0.069 Mold 332 1.8 2 0.21 1.0 2.6 0.033 1.5 — 0.05 0.070 1.1 0.002 2.10 0.035 Mold 336 1.8 3 0.23 1.0 2.3 0.054 1.6 — 0.00 0.043 1.0 — 2.00 0.054 Mold 200 3.3 4 0.23 1.0 2.2 0.013 1.6 — 0.02 0.030 1.0 — 2.00 0.013 DC — 10   5 0.21 1.1 2.2 0.051 1.6 0.045 0.01 0.030 1.0 0.047 2.10 0.143 DC — 10   6 0.21 1.0 2.3 0.430 1.6 — 0.00 0.050 1.1 — 2.10 0.430 Mold 200 3.3 7 0.20 1.2 2.5 0.033 1.4 0.033 0.05 0.070 1.1 0.002 2.30 0.068 Mold 393 1.1 8 0.20 1.2 2.5 0.033 1.4 0.033 0.05 0.070 1.1 0.002 2.30 0.068 Mold 326 1.9 9 0.24 1.0 2.3 0.220 1.5 — 0.00 0.045 1.0 — 2.00 0.220 Mold 200 3.3

TABLE 2 Microstructure Average Variation of equivalent distance Creep test Area ratio of Number circle between Creep determination intermetallic density of diameter of intermetallic character- Determination Grain compound intermetallic intermetallic compounds in ST Load istics criterion: diameter occupying in compound compound direction (standard Temperature stress Rupture rupture No. (μm) visual field (%) (piece/mm²) (μm) deviation) (° C.) (MPa) time (hr) LMP time ≥220 hrs 1 32.0 7.2 3400 4.0 2.13 180 220 250.2 10146 ∘ 2 36.0 6.6 3200 3.7 1.95 180 220 250.3 10147 ∘ 3 39.2 12.2 2200 4.2 2.00 180 220 265.3 10158 ∘ 4 42.5 9.0 5700 3.6 1.86 180 220 284.6 10172 ∘ 5 35.2 13.2 8300 3.7 2.02 180 220 570.9 10309 ∘ 6 44.6 10.4 6100 4.1 2.63 180 220 134.2 10024 x 7 33.9 17.6 5800 5.0 2.03 180 220 178.5 10080 x 8 29.9 19.4 7500 4.4 2.60 180 220 182.3 10084 x 9 38.8 9.5 5300 3.9 3.20 180 220 219.3 10120 x

Also, each underline in Table 1 and Table 2 expresses that the requirement is not fulfilled or the acceptance criteria are not fulfilled. “-” in Table 1 expresses that the composition is less than the detectable lower limit, and expresses that the mold temperature is not measured since the water-cooled mold is used in the DC casting method.

As shown in Table 1 and Table 2, the forged aluminum alloys related to No. 1 to No. 5 were excellent in the creep characteristics (all were the examples). Also, all of the forged aluminum alloys related to No. 1 to No. 5 had the tensile property fulfilling the JIS Standards.

With respect to the forged aluminum alloys related to No. 6 to No. 9, although the tensile strength fulfilled the JIS Standards, the creep characteristics were inferior (all were the comparative examples).

The forged aluminum alloy related to No. 1 to No. 9 having secured the result described above will be analyzed. FIG. 8 to FIG. 11 graphically show some items based on the result having been secured. FIG. 8 is a graph illustrating a relation between the total content of Mn, Cr, and Zr and the total content of Fe and Ni with respect to the example and the comparative example. FIG. 9 is a graph illustrating a relation between the average equivalent circle diameter of an intermetallic compound and the variation of the distance between intermetallic compounds in the ST direction with respect to the example and the comparative example. FIG. 10 is a graph illustrating a relation between the variation of the distance between intermetallic compounds in the ST direction and the rupture time of the creep test with respect to the example and the comparative example. FIG. 11 is a graph illustrating a relation between the average equivalent circle diameter of an intermetallic compound and the rupture time of the creep test with respect to the example and the comparative example. In all of FIG. 8 to FIG. 11, the example is plotted by “♦”, and the comparative example is plotted by “□”.

As shown in FIG. 8, in the example, it is found that both of the total content of Mn, Cr, and Zr and the total content of Fe and Ni are low total content. Also, as shown in FIG. 9, in the example, it is found that both of the average equivalent circle diameter of the intermetallic compound and the variation of the distance between intermetallic compounds in the ST direction have a low value. As shown in Table 1, Table 2, FIG. 10, and FIG. 11, it is found that both of the variation of the distance between intermetallic compounds in the ST direction and the average equivalent circle diameter of the intermetallic compound should have a low value in order that the rupture time of the creep test passes.

The photos of the optical microscope of each of the forged aluminum alloys related to No. 1 to No. 5 which were the example and No. 6 to No. 9 which were the comparative example were analyzed by the analysis software.

As a result, in the forged aluminum alloys related to No. 1 to No. 5, the intermetallic compounds were dispersed uniformly.

On the other hand, in the forged aluminum alloys related to No. 6 to No. 9, the intermetallic compounds were present to be adjacent and continuous with each other and were present to be apart from each other, and were dispersed non-uniformly.

Form the above, it could be confirmed that, in the forged aluminum alloys related to No. 1 to No. 5 which were the examples, the total content of Fe and Ni was 2.2 mass % or less, the total content of Mn, Cr, and Zr was 0.20 mass % or less, the average equivalent circle diameter of the intermetallic compound was 4.5 μm or less, and the variation of the distance between the intermetallic compounds in the ST direction was 2.3 or less. It was confirmed that the forged aluminum alloys related to No. 1 to No. 5 were allowed to be excellent in the creep characteristics by employing such configurations.

On the other hand, the forged aluminum alloys related to No. 6 to No. 9 did not fulfill some of the requirements with respect to the total content of Fe and Ni, the total content of Mn, Cr, and Zr, the average equivalent circle diameter of the intermetallic compound, and the variation of the distance between the intermetallic compounds in the ST direction. Therefore, the forged aluminum alloys related to No. 6 to No. 9 were inferior in the creep characteristics.

Also, with respect to the comparative examples, to be more specific, in the forged aluminum alloys related to No. 6, and No. 9, since the total content of Mn, Cr, and Zr exceeded 0.20 mass %, the variation of the distance between the intermetallic compounds in the ST direction came to exceed 2.3. Specifically, with respect to the forged aluminum alloys related to No. 6, and No. 9, the intermetallic compounds were not dispersed uniformly, and were distributed unevenly. Therefore, the forged aluminum alloys related to No. 6 and No. 9 resulted to be inferior in the creep characteristics.

Also, with respect to both of the forged aluminum alloys related to No. 7 and No. 8, the total content of Fe and Ni exceeded 2.2 mass %.

Here, with respect to the forged aluminum alloy related to No. 7, since the cooling rate of the molten metal was less than 1.2° C./sec, the average equivalent circle diameter of the intermetallic compound exceeded 4.5 μm. Therefore, the forged aluminum alloy related to No. 7 resulted to be inferior in the creep characteristics.

On the other hand, with respect to the forged aluminum alloy related to No. 8, since the cooling rate of the molten metal was 1.2° C./sec or more, although the average equivalent circle diameter of the intermetallic compound was 4.5 μm or less, the variation of the distance between the intermetallic compounds in the ST direction exceeded 2.3. With respect to the forged aluminum alloy related to No. 8, the intermetallic compounds were not dispersed uniformly, and were distributed unevenly. Therefore, the forged aluminum alloy related to No. 8 resulted to be inferior in the creep characteristics.

From the results on the forged aluminum alloys related to No. 7 and No. 8, it was found that the cooling rate of the molten metal largely affected the size of the intermetallic compound. Also, from these facts, it was presumed to be important to control the cooling rate of the molten metal appropriately.

Although the forged aluminum alloy and the manufacturing method for the forged aluminum alloy related to the present invention were explained above in detail by the embodiments and the examples, the gist of the present invention is not limited to them, and should be interpreted widely based on the claims. 

What is claimed is:
 1. A forged aluminum alloy, comprising: Si: 0.10-0.25 mass %; Fe: 0.9-1.3 mass %; Cu: 1.9-2.7 mass %; Mg: 1.3-1.8 mass %; Zn: 0.10 mass % or less; Ni: 0.9-1.2 mass %; and Ti: 0.01-0.1 mass %, with the balance being Al and inevitable impurities, wherein the total content of Fe and Ni is 2.2 mass % or less, the total content of Mn, Cr, and Zr is 0.20 mass % or less, the average equivalent circle diameter of an intermetallic compound is 4.5 μm or less, and variation of distance between the intermetallic compounds in the ST direction is 2.3 or less.
 2. A manufacturing method of a forged aluminum alloy having the alloy composition according to claim 1, comprising the steps of; cooling molten metal having the alloy composition with a cooling rate of 1.2° C./sec or more to cast an ingot; and manufacturing a forged aluminum alloy by subjecting the ingot to homogenization treatment, hot forging, solution treatment, quenching, and temper aging treatment. 